1. Field of the Invention
The present invention relates to a polymeric compound containing a glycopolymer, in particular a glycopolymer which is readily decomposed and the resulting decomposition products are readily recycled. In particular, the present invention relates to a polymeric compound containing an glycopolymer having a saccharide component, such as an oligosaccharide, which exclusively contains glucopyranose rings.
2. Description of the Related Art
These days, not only industrial waste but also domestic waste has become a prime consideration due to environmental pollution. Various resins used for industrial materials are not readily degradable or recyclable, and investigation and development of disposal methods having minimal adverse effects on the environment or novel materials applicable to such methods are in demand.
Examples of conventional methods for disposing of waste plastic having less adverse effects include a process comprising the steps of decomposing waste plastic into low-molecular compounds thermally or chemically, and then incinerating or burying the low-molecular compounds. However, incineration may contribute to earth warming, since it is accompanied by carbon dioxide discharge or cause s air pollution if the resins contain halogens, sulfur, or nitrogen. When buried, almost all resins practically used at the present time remain unchanged for long time-periods during which additives and the like disperse from the resins, thereby causing soil pollution.
Concerning the above problems, biodegradable polymeric compounds having few adverse effects on the earth's environment at the time of final disposal have actively been under development (e.g., Japanese Unexamined Patent Publication No. 5-287043). Biodegradable resins are generally classified into three types, i.e., (1) microorganism products, (2) natural products derived from plants, and (3) chemically synthesized products. An example of a microorganism product which is commercially available is "BIOPOLE" (Trade name) manufactured by MONSANTO Co. Ltd. "BIOPOLE" is a copolymerized polyester of D-3-hydroxybutylate and D-3-hydroxyvalerate derived from Alcaligenes Eutroplus, i.e., hydrogen bacteria and is biodegradable by microorganisms. Examples of natural products ar collagen, gelatin, starch, cellulose, and chitosan. These natural products per se are biodegradable. A mixture of starch and modified polyvinyl alcohol, a cellulose ester obtained by chemical modification of cellulose, and a complex of cellulose and chitosan are typical natural products. Examples of chemically synthesized biodegradable products are: water-soluble polymers such as polyvinyl alcohol and polyethylene glycol; and aliphatic polyesters such as polyethylene adipate and polycaprolactam.
Meanwhile, for efficient use of resources, waste plastic is decomposed into low-molecular weight compounds. The low molecular-weight decomposition products can be recycled as a starting material for forming polymeric compounds. For example, polystyrene is catalytically decomposed using a solid base catalyst and recovered as a styrene monomer or dimer and is supplied as a recycled material for polymerization into polystyrene. In another example, polyethylene terephthalate is decomposed to dimethyl terephthalate, ethylene glycol, or terephthalic acid by (1) methanolysis using methanol, (2) glycolysis using ethylene glycol, or (3) hydrolysis using an acid or base, respectively. These decomposition products can then be utilized as a starting material for synthesizing polyethylene terephthalate or other chemicals. However, according to the above examples, numerous purification steps are necessary to obtain recyclable components from the decomposition products. Such numerous steps increase the cost of recycling the decomposition products of waste plastic.
Although the above-mentioned biodegradable polymeric compounds are preferable when disposing by burying them as compared with conventional non-biodegradable resins such as polyethylene, polypropylene, and polyvinyl chloride, biodegradable polymeric compounds have never been known up to now which are synthesized with a view toward recycling their decomposition products.
A polymeric compound containing a saccharide, that is, a glycopolymer, which is synthesized by polymerization is described in detail, for example, in "High Polymers, Japan" (Vol. 45 (August), page 553 to 557, 1996). A glycopolymer synthesized by copolymerizing (a main-chain type) sucrose and a diester of adipic acid using an enzyme catalyst is described by D. R. Patil et al. in Table 1 on page 554 and from line 11 to 13 in the left column on page 555 of the above Journal. Furthermore, it is reported that this glycopolymer contains approximately 30 sucrose molecules, is decomposed by heat at approximately 150.degree. C., and is biodegradable in Macromolecules (D. R. Patil et al., Vol. 24, page 3462, 1991) and Biotechnology and Bioengineering (D. R. Patil et al., Vol. 37, page 639 to 646, 1991). However, the above literature does not disclose any glycopolymer in which an oligosaccharide exclusively having glucopyranose rings composes the main chain with a second component nor is ther any description suggesting that a biodegradable polymeric compound having excellent heat stability is obtained from the above compound.